Apache Canada Ltd. noticed the spill about 20 kilometres northeast of Zama City in a flyover June 1. The company told Alberta’s regulator, the Energy Resources Conservation Board, the same day, and notified a First Nations community and a trapper nearby shortly afterward “as a courtesy,” said board spokesman Bob Curran. But at the time it didn’t seem necessary to alert the public. “It wouldn’t have been required.”

It wasn’t until Apache reported the total volume of “produced water” spilled that the board issued a press release Wednesday.

“In this case it was very difficult to determine the actual spill volumes. It took a while for the company to delineate the affected area. … At the outset we did not know that the volumes were even close to that,” Curran said in an interview.

“We certainly had no indication that it was as large as it is.”

Much of the 42-hectare affected area is muskeg and sensitive wetlands, which made water depth and amount tough to measure, Curran said. “So it did take that time to get the volume nailed.”

Apache did not immediately return calls Tuesday. The Texas-based company said in a press release it’s investigating the spill’s cause.

Produced water is usually salty water that’s a byproduct of the oil and gas extraction process; it often contains hydrocarbons (such as oil), naturally occurring radioactive materials and other chemicals.

It doesn’t sound as dangerous as a crude oil spill might, but depending on the terrain and on the amount spilled, it can cause problems – contaminating soil, killing vegetation or infiltrating waterways.

Sidney Chambaud, a band councillor with the Dene Tha’, a local First Nations group, said that the spill has already contaminated water and killed trees and grass in the area. Dene Tha’ leaders are to meet Monday with Apache Canada Ltd. officials in Assumption, Alberta. Chambaud said that they want more information about what happened and what the company is going to do about it.

Whether the board notifies the public about produced water spills depends on how much is spilled and where it goes, Curran said.

“In this case, those volumes were substantial enough … to precipitate the news release.”

There were 23,484 of produced water spills in Alberta between Jan. 1, 1975 and Feb. 4, 2013. And according to the latest volume estimates, Apache’s Zama City spill is the 10th largest produced water spill since 1975.

The biggest was about 48-million litres, spilled from an Acclaim Energy oil well blowout just west of Edmonton in December, 2004. 436,000 litres of crude oil were released in that same spill, along with a substantial amount of gas. 500 people had to be evacuated from the area.

Apache Canada has been responsible for 949 spills in total during that time frame, of which 517 were produced water spills.

With files from the Canadian Press

Check out Alberta’s 10 biggest produced water spills in the interactive map below: